scholarly journals The influence of changes in flow regime caused by dam closure on channel planform evolution: insights from flume experiments

2021 ◽  
Vol 80 (4) ◽  
Author(s):  
Marcin Słowik ◽  
Kinga Kiss ◽  
Szabolcs Czigány ◽  
Alexandra Gradwohl-Valkay ◽  
József Dezső ◽  
...  
Keyword(s):  
Flow Regime ◽  
Sediment Load ◽  
Land Use Changes ◽  
Sediment Supply ◽  
River Channels ◽  
Flume Experiments ◽  
Sediment Delivery ◽  
Green River ◽  
Single Thread ◽  
Channel Planform

AbstractConstructions of dams influence river courses by cutting off sediment delivery and altering flow regime. We conducted flume experiments to study how sediment starvation, flow deficit and occurrence of series of floods with sediment load influence the evolution of channel planform. Results indicated that reduced flow caused a transition from a braided to incised single-thread planform with remains of inactive channels. The planform resembled rivers suffering from flow reduction caused by dams and constructions of irrigation canals (e.g., Central Platte River, USA). Simulation of series of floods with no sediment delivery caused the formation of an anabranching planform with incised main channel, alluvial islands, and side channels active at high flows. This evolution corresponds to river courses altered by sediment starvation and series of floods (e.g., the lower Drava River, Hungary). Floods with delivery of fines created a single-thread, incised channel with terraces along banks. Such channels are formed by rivers closed by dams, and compensating sediment deficit by sediment load from tributaries and/or floodplains (e.g., the Green River, USA). The flume tests demonstrated how flow and sediment deficit influence river channels and that the potential for restoration strongly depends on possibilities for the activation of sediment supply. However, restoration of pre-dam conditions is often impossible owing to other hydro technical works and land use changes. The highest restoration potential refers to the rivers compensating sediment delivery blocked by dams with tributary and floodplain resources.

Dynamics of sediment delivery to mangrove forests of the Ganges-Brahmaputra-Meghna Delta

2020 ◽  
Author(s):  
Richard Hale ◽  
Alexandra Garnand ◽  
Carol Wilson
Keyword(s):  
Sediment Load ◽  
Monsoon Season ◽  
Sediment Concentration ◽  
Water Velocity ◽  
Sediment Supply ◽  
Tidal Range ◽  
Mangrove Forests ◽  
Southern Coast ◽  
Sediment Delivery ◽  
The Ganges

<p>The Ganges-Brahmaputra-Meghna Delta (GBMD) is among the largest in the world, nourished by the ~1 Gt/yr sediment load of its titular rivers. Approximately 75% of this sediment load is debouched to the Bay of Bengal, with ~180 Mt subsequently reworked by tidal processes across the southwestern portion of the delta. This region includes this Sundarbans National Reserve Forest (SNRF), which is the words’ largest continuous mangrove stand. In addition to global sea level rise and the enhanced subsidence intrinsic to deltas, ongoing and proposed alterations to the upstream fluvial sediment supply threaten the future viability of this important ecological and cultural resource.</p><p> </p><p>In this study, we use data collected in situ by acoustic and optical instrumentation to examine the physical processes controlling sedimentation in the mangrove forest along the southern coast during both the monsoon (October 2019) and dry seasons (March 2020).  These data are then compared with sedimentation rates measured using sediment elevation tables and marker horizons, as well as observations made 100 km further inland near the northern extent of the SNRF. At this inland site, sediment supply, inundation depth, and salinity have been identified as important factors controlling sediment deposition to the mangrove platform, which ranges from ~1 cm during the dry season (November – June), to > 2 cm during the monsoon (July-October). Data from the second location along the coast are vital for understanding the regional nature of the various threats to delta viability.</p><p> </p><p>Preliminary analysis of the 2019 monsoon season data from the southern coast reveals the relative importance of water depth, water velocity, and mangrove pneumatophore density on modulating both water velocity and suspended sediment concentration. Previous studies have identified that while the inland location features a larger tidal range (~5 m vs. ~3 m), frequent cyclone activity likely impacts sedimentation at the coastal site. Data collected in March 2020 will address how these variables impact controls on sedimentation both seasonally and regionally. Results from this study demonstrate the importance of providing regional context to sedimentation studies, as delta communities adapt to dynamic forcing conditions.   </p>

A multi-temporal inventory for constraining earthflow source-to-sink pathways in the Sillaro River basin, Northern Apennines

2020 ◽  
Author(s):  
Sharon Pittau ◽  
Matteo Berti ◽  
Giovanna Daniele ◽  
Marco Pizziolo ◽  
Francesco Brardinoni
Keyword(s):  
River Basin ◽  
Rainfall Variability ◽  
Land Use Changes ◽  
Relevant Information ◽  
Quantitative Information ◽  
Northern Apennines ◽  
Sediment Supply ◽  
Sediment Delivery ◽  
Source To Sink ◽  
Multi Temporal

<p>In mountain environments, landslide sediment supply is one of the main factors that can affect fluvial morphodynamics. In settings underlain by clay-rich lithologies, where earthflows are the dominant agents of hillslope sediment transfer, limited quantitative information is available on the contribution of these processes to the sediment budget. This is a critical aspect both for addressing basic scientific questions on landscape evolution, as well for tackling more applied issues on river sediment management.</p><p>This study focuses on the mountain portion of the Sillaro River basin (138 km<sup>2</sup>), a fluvial system underlain by argillites and siltstones of the Ligurian domain, Northern Apennines (Italy). Here, earthflows are the most common landslide type. Through the compilation of a multi-temporal earthflow inventory (1954-2019), we aim to: (i) characterize earthflow source-to-sink sedimentary pathways, with special reference to sediment delivery to ephemeral and perennial streams; (ii) explore possible litho-topographic controls on earthflow size, frequency and recurrence; (iii) examine historical trend of earthflow activity in relation to rainfall variability and land use changes. Finally, the high and extended temporal resolution of the inventory, will offer the opportunity to test how relevant information could complement the existing inventory of the Emilia-Romagna region, for evaluating earthflow hazard and risk potential.</p><p>Data collection entailed inspection of 12 sequential aerial photo sets (1954, 1969, 1976, 1988, 1996, 2000, 2006, 2008, 2011, 2014, 2016, and 2018), through which earthflows were classified and mapped in GIS environment. This remotely-based activities were complemented by confirmatory field visits on a subset of most recent events. Overall, we have mapped a total of 506 earthflows, which collectively extend over an area of 4.1 km<sup>2</sup>.</p><p>Preliminary results show that earthflow size (i.e., total disturbed area) ranges from 400 m<sup>2</sup> to 98000 m<sup>2</sup>, with frequencies peaking around 10000 m<sup>2</sup>. In terms of source-to-sink pathways, we find that earthflows chiefly tend to deliver sediment to ephemeral gully channels (61%) and perennial tributaries (25%). Whereas, 5% of the events remain on the slopes, and another 5% are buffered by roads and similar anthropogenic barriers. Only a very limited proportion of earthflows (4%) makes it directly to the Sillaro River main stem.</p><p>This work, as part of the projects BEDFLOW and BEFLOW PLUS, is partially funded by Fondazione Cassa di Risparmio in Bologna.</p>

scholarly journals Amplified Impact of Climate Change on Fine-Sediment Delivery to a Subsiding Coast, Humboldt Bay, California

2021 ◽  
Author(s):  
Jennifer A. Curtis ◽  
Lorraine E. Flint ◽  
Michelle A. Stern ◽  
Jack Lewis ◽  
Randy D. Klein
Keyword(s):  
Climate Change ◽  
Climate Change Impacts ◽  
Baseline Period ◽  
Climate Impacts ◽  
Sediment Supply ◽  
Balance Model ◽  
Climate Projections ◽  
Sediment Delivery ◽  
Beneficial Reuse ◽  
Coastal Margin

AbstractIn Humboldt Bay, tectonic subsidence exacerbates sea-level rise (SLR). To build surface elevations and to keep pace with SLR, the sediment demand created by subsidence and SLR must be balanced by an adequate sediment supply. This study used an ensemble of plausible future scenarios to predict potential climate change impacts on suspended-sediment discharge (Qss) from fluvial sources. Streamflow was simulated using a deterministic water-balance model, and Qss was computed using statistical sediment-transport models. Changes relative to a baseline period (1981–2010) were used to assess climate impacts. For local basins that discharge directly to the bay, the ensemble means projected increases in Qss of 27% for the mid-century (2040–2069) and 58% for the end-of-century (2070–2099). For the Eel River, a regional sediment source that discharges sediment-laden plumes to the coastal margin, the ensemble means projected increases in Qss of 53% for the mid-century and 99% for the end-of-century. Climate projections of increased precipitation and streamflow produced amplified increases in the regional sediment supply that may partially or wholly mitigate sediment demand caused by the combined effects of subsidence and SLR. This finding has important implications for coastal resiliency. Coastal regions with an increasing sediment supply may be more resilient to SLR. In a broader context, an increasing sediment supply from fluvial sources has global relevance for communities threatened by SLR that are increasingly building resiliency to SLR using sediment-based solutions that include regional sediment management, beneficial reuse strategies, and marsh restoration.

scholarly journals An Innovative Approach to Minimizing Uncertainty in Sediment Load Boundary Conditions for Modelling Sedimentation in Reservoirs

Water ◽  
2018 ◽  
Vol 10 (10) ◽  
pp. 1411 ◽  
Author(s):  
Sardar Ateeq-Ur-Rehman ◽  
Minh Bui ◽  
Shabeh Hasson ◽  
Peter Rutschmann
Keyword(s):  
Boundary Conditions ◽  
Sediment Load ◽  
Sediment Supply ◽  
Coefficient Of Determination ◽  
Computational Time ◽  
Indus River ◽  
Sediment Deposits ◽  
Subsequent Decrease ◽  
Delta Formation ◽  
Near Future

A number of significant investigations have advanced our understanding of the parameters influencing reservoir sedimentation. However, a reliable modelling of sediment deposits and delta formation in reservoirs is still a challenging problem due to many uncertainties in the modelling process. Modelling performance can be improved by adjusting the uncertainty caused by sediment load boundary conditions. In our study, we diminished the uncertainty factor by setting more precise sediment load boundary conditions reconstructed using wavelet artificial neural networks for a morphodynamic model. The model was calibrated for hydrodynamics using a backward error propagation method. The proposed approach was applied to the Tarbela Reservoir located on the Indus River, in northern Pakistan. The results showed that the hydrodynamic calibration with coefficient of determination (R2) = 0.969 and Nash–Sutcliffe Efficiency (NSE) = 0.966 also facilitated good calibration in morphodynamic calculations with R2 = 0.97 and NSE = 0.96. The model was validated for the sediment deposits in the reservoir with R2 = 0.96 and NSE = 0.95. Due to desynchronization between the glacier melts and monsoon rain caused by warmer climate and subsequent decrease of 17% in sediment supply to the Tarbela dam, our modelling results showed a slight decrease in the sediment delta for the near future (until 2030). Based on the results, we conclude that our overall state-of-the-art modelling offers a significant improvement in computational time and accuracy, and could be used to estimate hydrodynamic and morphodynamic parameters more precisely for different events and poorly gauged rivers elsewhere in the world. The modelling concept could also be used for predicting sedimentation in the reservoirs under sediment load variability scenarios.

Modeling land use changes and their impact on sediment load in a Mediterranean watershed

CATENA ◽  
2018 ◽  
Vol 163 ◽  
pp. 342-353 ◽  
Author(s):  
Giovanni Romano ◽  
Ossama M.M. Abdelwahab ◽  
Francesco Gentile
Keyword(s):  
Land Use ◽  
Sediment Load ◽  
Land Use Changes

Effects of episodic sediment supply on experimental step-pool channel morphology, bedload transport and channel stability

2021 ◽  
Author(s):  
Jiamei Wang ◽  
Marwan A. Hassan ◽  
Matteo Saletti ◽  
Xingyu Chen ◽  
Xudong Fu ◽  
...  
Keyword(s):  
Grain Size ◽  
Morphological Changes ◽  
Mass Movement ◽  
Channel Morphology ◽  
Bedload Transport ◽  
Sediment Supply ◽  
Primary Control ◽  
Channel Stability ◽  
Flume Experiments ◽  
Small Magnitude

<p>Steep step-pool streams are often coupled to adjacent hillslope, directly receiving episodic sediment supply from mass movement processes such as landslides and debris flows. The response of step-pool channels to the variations in sediment supply remains largely unexplored. We conducted flume experiments with a poorly sorted grain-size distribution in an 8%-steep, 5-m long flume with variable width at the University of British Columbia, to study the effects of episodic sediment supply on channel evolution. After a conditioning phase with no feed, the channel was subjected to sediment pulses of different magnitude and frequency under constant flow discharge. High-resolution data of hydraulics, bedload transport, bed surface grain size, and channel morphology were collected every 10-20 minutes and an additional time at the end of each pulse.</p><p>In response to sediment pulses, we recorded an increase in bedload transport rates, channel aggradation, bed surface fining, and continuous step formation and collapse. In between pulses, bedload rates dropped by several orders of magnitude, net erosion occurred, the bed surface gradually coarsened, and steps became more stable. The small-magnitude high-frequency pulses caused smaller but more frequent spikes in bedload transport, bed surface evolution, and thus step stability. Instead, the large-magnitude low-frequency pulses cause larger changes but provided a longer time for the channel to recover. This suggests that in step-pool channels pulse magnitude is a key control on channel rearrangement, while pulse frequency controls how fast and strong the recovery is.</p><p>The frequency and stability of steps varied as a function of local channel width, showing that channel geometry is a primary control on step formation and stability even under episodic sediment supply conditions. Instead, the effect of sediment pulses is less important because the total number and average survival time of steps were similar among runs with different pulses. The critical Shields stress decreased following sediment pulses, then increased immediately after, and fluctuated until the next pulse. The variations in sediment supply caused cycles in bedload transport rate, surface and bedload texture, thus controlling the variability in the threshold for motion.</p><p>Our results indicate that episodic sediment supply is a primary control on the evolution of step-pool channels, with sediment feed magnitude affecting mostly morphological changes, and sediment feed frequency controlling channel stability.</p>

scholarly journals Paleogeographic evolution and avulsion history of the Holocene Rhine-Meuse delta, The Netherlands

2002 ◽  
Vol 81 (1) ◽  
pp. 97-112 ◽  
Author(s):  
H.J.A. Berendsen ◽  
E. Stouthamer
Keyword(s):  
Large Scale ◽  
Sediment Load ◽  
Geographical Information ◽  
River Channels ◽  
Human Influence ◽  
Channel Pattern ◽  
The Holocene ◽  
History Of ◽  
Peat Formation ◽  
River Banks

AbstractApproximately 200,000 lithological borehole descriptions, 1200 14C dates, 36,000 dated archaeological artifacts, and gradients of palaeochannels were used to reconstruct the Holocene evolution of the fluvial part of the Rhine-Meuse delta. Ages of all Holocene channel belts were stored in a Geographical Information System database that enables generation of palaeogeographic maps for any time during the Holocene. The time resolution of the palaeogeographic reconstruction is about 200 years.During the Holocene, avulsion was an important process, resulting in frequent shifts of areas of clastic sedimentation. Palaeogeographic evolution and avulsion history of the Rhine-Meuse delta are governed by complex interactions among several factors. These are: (1) Location and shape of the Late Weichselian palaeovalley. In the Early Holocene, rivers were confined to the LateWeichselian valley. When aggradation shifted upstream, the margins of the valley were crossed by newly formed channel belts. (2) Sealevel rise, which resulted in back-filling of the palaeovalley. (3) River channel pattern. In the central-western part of the delta, a straight anastomosed channel pattern with large-scale crevassing developed as a result of sealevel rise and the associated decrease of stream power. (4) Neotectonics. Differential tectonic movements of the Peel Horst and Roer Valley Graben seem to have influenced river behaviour (formation of an asymmetrical meander belt, location of avulsion nodes in fault zones), especially from 4500–2800 14C yr BP when the rate of sealevel rise had decreased. After 2800 14C yr BP sealevel rise further decreased, and tectonic influence still may have influenced avulsions, but from then on other factors became dominant. (5) Increased discharge, sediment load and/or within-channel sedimentation. After 2800 14C yr BP, meander wavelenghts increased, which is interpreted as a result of increased bankfull discharge and/or within channel sedimentation. After 2000 14C yr BP both discharge and sediment load increased as a result of human influence. (6) Coastal configuration. The limited number of tidal inlets and extensive peat formation restricted the number of avulsions in the western part of the delta, and enhanced channel reoccupation. (7) Composition of the substrate and river banks. Meandering river channels tended to adhere to the sandy margins of the LateWeichselian palaeovalley, and high channel sinuosity is found in areas where river banks consisted of sand. Peat formation, which was most extensive in the western part of the back-barrier area especially between 4000 and 3000 14C yr BP, more or less fixed the river pattern at that time, hampering avulsions. (8) Marine ingressions, e.g. the 1421 AD St. Elizabeth’s flood caused large-scale erosion in the southwestern part of the fluvial deltaic plain, resulting in a shift of the main drainage to the SW. (9) Human influence. Since about 1100 AD human influence dominated the palaeogeographic evolution. Rivers were embanked and natural avulsions did no longer occur.

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